Tumor growth and metastasis are dependent on neovascularization. By understanding the mechanisms that control the neovascular response, it may be possible to design therapeutic strategies to selectively prevent of halt pathological growth of vessels and consequently restrain the progression of solid tumors. Thrombospondin-1 (TSP1), a glycoprotein originally described as a major component of platelet alpha-granules, has been implicated in the negative regulation of angiogenesis. This glycoprotein is expressed by endothelial cells late in development and inhibits angiogenic progression both in vivo and in vitro. TSP1 has also been acknowledged as a tumor-suppressor gene in human-hamster cell hybrids and has been recognized in the inhibition of the tumorigenic ability of MCF-7 breast cancer cells. Recently, we found that TSP1 might also play a role in the suppression of physiological angiogenesis during the human endometrial cycle. Expression of TSP1 in the human endometrium corresponded temporally to the cessation of vascular outgrowth. TSP1 protein and mRNA were detected in the secretory, but not in the proliferative phase of the human endometrium in 16 cases examined. Interestingly, TSP1 has also been identified in other steroid-dependent tissues, including the mammary gland, and in steroid-dependent tumors. We have now demonstrated that TSP1 mRNA is stimulated by progesterone and by estrogen. In the current project, we propose to define steroid- responsive elements which might be involved in the regulation of TSP1 expression during angiogenesis. Specifically, we will (1) test whether the effect of progesterone and estrogen on TSP1 transcription is associated with initiation or stabilization of transcripts, (2)define the cis-acting elements involved in the steroid-response, and (3) identify nuclear proteins which interact with the cis-acting sequences. Clearly, the regulation of TSP1, expression by steroids is an unexplored, but potentially important avenue that could have implications for the clinical management of vascular growth. A second facet of this proposal will evaluate the efficacy of TSP1 as an angiostatic mediator in solid tumors. We will test the effect of TSP1 on vascular growth of mammary tumors in the MMTV c-neu transgenic mouse. Initially, the distribution and the endogenous levels TSP1 mRNA and protein will be assessed in mammary tumors that develop spontaneously in these mice. Subsequently, slow-release delivery to TSP1 will be accomplished by subcutaneous implants and the extension of new vessels will be assessed by three dimensional reconstruction. Finally, we will examine the kinetics of endothelial cell growth and vascular regression in neoplastic areas of both control and treated animals. Understanding the mechanism by which TSP1 regulates blood vessel formation is of considerable interest and represents the long-range goal of this proposal.